The method developed herein is more suitable for on-site detection
due to its simple procedure and low cost. Particularly, the
initial screening capability demonstrated above (Fig. 5b) provides
either a positive or negative confirmation with a regular
smartphone and a free app. Without further optimization, a
detection limit of 0.2 μg mL−1 (∼200 ppb, also as the threshold
value for qualitative detection) was achieved with this smartphone-
compatible colorimetric assay, meeting the requirements
of the FAO and WHO